The field of the invention relates generally to a vehicle time based management system, and more specifically, to a method and systems for vertical navigation using time-of-arrival control.
Conventionally, aircraft are controlled in three dimensions; latitude, longitude, and altitude. More recently, the ability to control aircraft in the fourth dimension, time, has been shown to enable advanced airspace management resulting in increased capacity. The use of time-based arrival management facilitates earlier landing time assignments and more efficient use of the runway. This also results in economic benefits if each aircraft can determine its desired landing time using its most fuel optimum flight profile. However, in the absence of a defined geometric descent profile, current vertical navigation control algorithms use laws that control the elevators to a predetermined vertical path or vertical speed while maintaining a fixed throttle setting (typically idle). Using this control method, the speed is allowed to fluctuate over a large range of values, resulting in varying and inaccurate Estimated Time-of-Arrivals (ETAs) at points downstream of the aircraft. This adversely impacts the aircraft's adherence to a time constraint, typically referred to as a Required Time-of-Arrival (RTA) or Controlled Time-of-Arrival (CTA).
An aircraft descent trajectory is typically constructed by an onboard Flight Management System (FMS) backward from the destination to the point where the descent begins—referred to as the Top of Descent (T/D). The vertical portion of this computed trajectory consists of three general portions:
1) Approach Segment—this is the lowest portion of the descent, and contains a deceleration to the final landing speed along with extensions of high-lift devices and landing gear.
2) Geometric Segment—this is the middle portion of the descent, and is computed as a geometric sequence of lines which attempt to honor all altitude constraints. This segment may not exist if there are no altitude constraints that require it.
3) Idle Segment—this is the upper portion of the descent, and is computed assuming the descent target speed and idle thrust. Estimated (“forecast”) winds and temperatures are assumed in the computation of this segment.
When the aircraft is flying the idle segment of the trajectory, the throttle is fixed at an idle setting and an algorithm controls the elevators to the predefined vertical path (VPATH). In this control strategy, speed is allowed to fluctuate. When the estimated parameters used to construct the descent path, most notably winds and temperatures, match the actual parameters, the speed of the aircraft will match the intended target speed. However, it is more likely the estimated parameters will vary from the actual values encountered in flight, and, in turn, cause the speed of the aircraft to deviate from the target airspeed.
A traditional vertical navigation strategy for idle segments will allow the actual airspeed to deviate from the target airspeed by some preset value (a typical value is 15 knots). When the deviation exceeds the preset threshold, the system will attempt to add thrust or drag to zero the difference between the actual airspeed and the target airspeed. For an underspeed condition, the system will attempt to add thrust, either by placing the throttle in a speed control mode (A/T engaged) or by prompting the flight crew. For an overspeed condition, the system will attempt to add drag either automatically or by prompting the flight crew; most systems today do not support the automatic addition of drag. The original purpose of this design was to ensure that the actual airspeed did not exceed the performance limits of the aircraft and/or speed constraints imposed by the crew, navigation procedures, or aviation authorities. The use of relatively large speed margins around the target speed was driven by a desire to minimize mode transitions while satisfying the speed constraints and limits. However, allowing the speed to fluctuate by relatively large speed margins makes it very difficult to accurately meet a time constraint ahead of the aircraft.
An alternative method has been recently proposed by US Patent Application US 2005/0283306. In this method, the vertical navigation control strategy is to retain the idle thrust setting and use the elevators to control to speed as long as the actual aircraft altitude is within some range of the specified vertical path position at the current lateral position. When the actual altitude deviates by more than this value, the control strategy is modified to regain the specified vertical path while maintaining the target speed. Unfortunately, this method will also have a negative effect on the time-of-arrival control if the altitude band is too large as the ground speed (which directly affects time-of-arrival) is dependant not just on airspeed but also on altitude. Conversely, if the altitude band is too small, the pitch of the aircraft may continually vary and negatively impact the comfort of the aircraft passengers. It should be noted that this method does not truly address the energy problem in situations where additional drag is required to deal with errors in forecasted parameters. It simply allows the error to manifest itself as an altitude error rather than a speed error. It does not truly solve the four-dimensional Required Navigation Performance (4D RNP) problem.
Another alternative method has been proposed by U.S. Pat. No. 6,507,782. This patent promotes the construction of a descent path which replaces idle path segments with shallower descent path segments. Since the path is shallower than idle, the throttle can be used to control to speed in most circumstances, thus increasing the ability of the control system to satisfy a time constraint in descent. Two methods for construction of a path shallower than idle are suggested: (1) construct the descent segment assuming speed on the elevator but use idle thrust plus some throttle increment (Idle+Δ) rather than pure idle thrust or (2) replace an idle segment with a constant flight path angle (FPA) shallower than idle. Both these ideas are not particularly new, and both have their relative disadvantages. For example, the Airbus A320/A340 FMS has the concept of an Idle+Δ thrust path for constant speed idle segments. The intent of the A320/A340 design was to add some speed margin to allow the FMS to control speed deviations automatically via the throttle as speedbrakes require crew intervention. The problem with the Idle+Δ concept as presented in the patent is that, like the A320/A340 design, it must be empirically derived and stored in the performance database for each aircraft. It also results in a somewhat static speed margin as compared to a modifiable FPA. Likewise, the constant FPA approach is an idea that has been presented in industry and implemented as part of a past R&D program. The problems with this approach include: (a) it may result in longer decelerations, (b) it may be very expensive in terms of fuel usage, and (c) it may be difficult to find a single, reasonable FPA that works for a range of conditions of a given aircraft.